Who accessed the voicemails?

> Detectives .. have since uncovered evidence that showed the girl's phone would automatically delete voicemails 72 hours after being listened to. Some of those messages were deleted prior to anyone working for the NotW gaining access to Milly's voicemails, Scotland Yard reportedly said ...

Regardless, NotW did access the voicemail and any voicemails so accessed would be deleted after 72 hours, assuming this to be true. Also, they would have a record of the phone numbers used to access the voicemails, wouldn't they? Just how difficult is it for security services to trace a phone call.